tests: skip demandimport if disabled
demandimport and setuptools and decorator (from ironpython) and
pygments leads to lots of fail.
If demandimport is disabled we should skip testing it...
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL